Nurgle Daemon psykers have access to three psychic powers that cause direct damage:

Smite (warp charge 5) targets one enemy unit within 18", dealing D3 mortal wounds, or D6 if the test result is 11+.

Stream of Corruption (warp charge 5) targets the closest enemy unit within 7", dealing D3 mortal wounds if the unit has less then 10 models, and D6 mortal wounds if it has 10+ models.

Nurgle's Rot (warp charge 7) targets all non-Nurgle units within 7", dealing D3 mortal wounds for every 4+ rolled on a D6 for each unit.

The question we ask is, "Which is best?" And naturally the answer is, "It depends." Here are the details...

All three deal mortal wounds to one enemy unit. The key metric therefore is the expected number of wounds inflicted by each power.

Smite has a 27/36 chance of D3 (mean 2) wounds, and a 3/36 chance of D6 (mean 3.5) wounds. That gives an expected number of 1.791(6) mortal wounds: our base value for comparison.

Stream of Corruption has a 30/36 chance of manifesting an average of 2 wounds for 9- models (expected 1.(6) wounds), and 3.5 wounds for 10+ models (expected 2.91(6) wounds). So Stream of Corruption is better than Smite only if the target unit contains 10 or more models. (Also it has a shorter range, and you don't get to pick which unit is targeted.)

Nurgle's Rot has a 21/36 chance of manifesting an average of 1 wound per unit (50% chance of D3 wounds per unit). So Nurgle's Rot is better than Smite only if there are four or more enemy units within 7". (Also it has a shorter range, and will affect friendly non-Nurgle units.)

Comparing Stream of Corruption with Nurgle's Rot, it's a trade-off between number of models in a unit and number of units within range. Specifically, Stream of Corruption is better than Nurgle's Rot for a small unit (9 or fewer models) if there are only 1 or 2 units within 7". Stream of Corruption is exactly as good as Nurgle's Rot for a large unit (10 or more models) if there 5 units within 7" (and so, better if there are fewer than 5 units within 7").

Here's a handy diagram of which power is best (ignoring range and targeting limitations):

Bileblade

The most important complication to factor into our calculations is the Bileblade. This Great Unclean One weapon (if selected instead of a Plague Flail) add 1 to a GUO's Psychic test at the cost of a mortal wound (potentially ignored by Disgustingly Resilient).

If the Bileblade's ability is used:

Smite has a 27/36 chance of D3 (mean 2) wounds, and a 6/36 chance of D6 (mean 3.5) wounds. That gives an expected number of 2.08(3) mortal wounds.

Stream of Corruption has a 33/36 chance of manifesting. That gives an expected number of 1.8(3) mortal wounds for a unit of 9- models, and 3.208(3) for a unit of 10+ models; still comparable to Smite in the same way.

Nurgle's Rot has a 26/36 chance of manifesting. This makes it better than Smite if there are 2+ units within 7". And this makes it better than Stream of Corruption for 3+ units compared to one small unit, and 5+ units compared to one large unit.